Rivet analyses

title: BESIII_2016_I1384778

Azimuthal asymmetries in inclusive charged pion-pair production at $\sqrt{s}=3.65$ GeV

Experiment: BESIII (BEPC)

Inspire ID: 1384778

Status: VALIDATED

Authors: - Peter Richardson

References: - Phys.Rev.Lett. 116 (2016) no.4, 042001

Beams: e+ e-

Beam energies: (1.8, 1.8)GeV

Run details: - e+e- to hadrons

Measurement of azimuthal asymmetries in inclusive charged pion-pair production at $\sqrt{s}=3.65$ GeV by the BESII experiment

Source code:BESIII_2016_I1384778.cc

```c++ // -- C++ --

include "Rivet/Analysis.hh"

include "Rivet/Projections/Beam.hh"

include "Rivet/Projections/FinalState.hh"

include "Rivet/Tools/Random.hh"

namespace Rivet {

/// @brief Collins assymmetry class BESIII_2016_I1384778 : public Analysis { public:

/// Constructor
RIVET_DEFAULT_ANALYSIS_CTOR(BESIII_2016_I1384778);


/// @name Analysis methods
/// @{

/// Book histograms and initialise projections before the run
void init() {
  declare(Beam(), "Beams");
  declare(FinalState(Cuts::abspid == PID::PIPLUS), "FS");
  // book the histograms
  _h_L.resize(6);
  _h_U.resize(6);
  _h_C.resize(6);
  for (size_t ix = 0; ix < _h_L.size(); ++ix) {
    const string pre = "/TMP/h_z1z2_" + to_string(ix + 1);
    book(_h_L[ix], pre + "_L", 20, 0., M_PI);
    book(_h_U[ix], pre + "_U", 20, 0., M_PI);
    book(_h_C[ix], pre + "_C", 20, 0., M_PI);
  }
  book(_h_pT_L, {0., 0.2, 0.3, 0.45, 0.8, 1.4});
  book(_h_pT_U, {0., 0.2, 0.3, 0.45, 0.8, 1.4});
  book(_h_pT_C, {0., 0.2, 0.3, 0.45, 0.8, 1.4});
  for (size_t ix = 1; ix < _h_pT_L->numBins() + 1; ++ix) {
    const string pre = "/TMP/h_pT_" + to_string(ix);
    book(_h_pT_L->bin(ix), pre + "_L", 20, 0.0, M_PI);
    book(_h_pT_U->bin(ix), pre + "_U", 20, 0.0, M_PI);
    book(_h_pT_C->bin(ix), pre + "_C", 20, 0.0, M_PI);
  }
}


/// Perform the per-event analysis
void analyze(const Event& event) {
  // get the axis, direction of incoming electron
  const ParticlePair& beams = apply<Beam>(event, "Beams").beams();
  Vector3 axis;
  if (beams.first.pid() > 0) {
    axis = beams.first.momentum().p3().unit();
  }
  else {
    axis = beams.second.momentum().p3().unit();
  }
  // loop over pions pair, using index to avoid double counting
  Particles pions = apply<FinalState>(event, "FS").particles();
  for (unsigned int i1 = 0; i1 < pions.size(); ++i1) {
    const double x1 = 2. * pions[i1].momentum().t() / sqrtS();
    // cut on z1
    if (x1 < 0.2 || x1 > 0.9) continue;
    // cos theta cut
    if (abs(cos(pions[i1].momentum().p3().polarAngle())) > 0.93) continue;
    for (unsigned int i2 = i1 + 1; i2 < pions.size(); ++i2) {
      // cut on z2
      const double x2 = 2. * pions[i2].momentum().t() / sqrtS();
      if (x2 < 0.2 || x2 > 0.9) continue;
      // cos theta cut
      if (abs(cos(pions[i2].momentum().p3().polarAngle())) > 0.93) continue;
      // cut on opening angle (>120 degrees)
      if (pions[i1].momentum().p3().angle(pions[i2].momentum().p3()) > 2. * M_PI / 3.) {
        continue;
      }
      Particle p1 = pions[i1], p2 = pions[i2];
      double z1(x1), z2(x2);
      // randomly order the particles
      if (rand01() < 0.5) {
        swap(p1, p2);
        swap(z1, z2);
      }
      // particle 2 defines the z axis
      Vector3 ez = p2.momentum().p3().unit();
      // beam and 2 define the plane (y is normal to plane)
      Vector3 ey = ez.cross(axis).unit();
      // x by cross product
      Vector3 ex = ey.cross(ez).unit();
      // phi
      double phi = ex.angle(p1.momentum().p3());
      // hists vs z1,z2
      unsigned int ibin = 0;
      if (z1 <= .3 && z2 <= .3) {
        ibin = 0;
      }
      else if (z1 > 0.5 && z2 > 0.5) {
        ibin = 5;
      }
      else if (min(z1, z2) <= 0.3) {
        if (max(z1, z2) > 0.5) {
          ibin = 2;
        }
        else {
          ibin = 1;
        }
      }
      else {
        if (max(z1, z2) > 0.5) {
          ibin = 4;
        }
        else {
          ibin = 3;
        }
      }
      _h_C[ibin]->fill(phi);
      if (p1.pid() == p2.pid()) {
        _h_L[ibin]->fill(phi);
      }
      else {
        _h_U[ibin]->fill(phi);
      }
      // hists vs pT
      double pPar2 = sqr(ez.dot(p1.momentum().p3()));
      double pPerp = sqrt(p1.momentum().p3().mod2() - pPar2);
      _h_pT_C->fill(pPerp, phi);
      if (p1.pid() == p2.pid()) {
        _h_pT_L->fill(pPerp, phi);
      }
      else {
        _h_pT_U->fill(pPerp, phi);
      }
    }
  }
}

pair<double, double> calcAsymmetry(Estimate1DPtr hist) {
  double sum1(0.), sum2(0.);
  for (const auto& b : hist->bins()) {
    double Oi = b.val();
    if (Oi == 0. || std::isnan(Oi)) continue;
    double ai = 1.;
    double bi = 0.5 * (sin(2. * b.xMax()) - sin(2. * b.xMin())) / b.xWidth();
    double Ei = b.errAvg();
    sum1 += sqr(bi / Ei);
    sum2 += bi / sqr(Ei) * (Oi - ai);
  }
  return make_pair(sum2 / sum1, sqrt(1. / sum1));
}


/// Normalise histograms etc., after the run
void finalize() {
  // ratios
  BinnedEstimatePtr<string> _h_z_UL, _h_z_UC;
  book(_h_z_UL, 1, 1, 5);
  book(_h_z_UC, 1, 1, 6);
  for (size_t ix = 0; ix < _h_L.size(); ++ix) {
    normalize(_h_L[ix]);
    normalize(_h_U[ix]);
    normalize(_h_C[ix]);
    const string pre = "/TMP/R_z1z2_" + to_string(ix + 1);
    Estimate1DPtr R1;
    book(R1, pre + "_UL", 20, 0., M_PI);
    divide(_h_U[ix], _h_L[ix], R1);
    Estimate1DPtr R2;
    book(R2, pre + "_UC", 20, 0., M_PI);
    divide(_h_U[ix], _h_C[ix], R2);
    pair<double, double> asym1 = calcAsymmetry(R1);
    _h_z_UL->bin(ix + 1).set(asym1.first, asym1.second);
    pair<double, double> asym2 = calcAsymmetry(R2);
    _h_z_UC->bin(ix + 1).set(asym2.first, asym2.second);
  }
  Estimate1DPtr _h_pT_UL, _h_pT_UC;
  book(_h_pT_UL, 2, 1, 4);
  book(_h_pT_UC, 2, 1, 5);
  for (size_t ix = 1; ix < _h_pT_L->numBins() + 1; ++ix) {
    normalize(_h_pT_L->bin(ix));
    normalize(_h_pT_U->bin(ix));
    normalize(_h_pT_C->bin(ix));
    const string pre = "/TMP/R_pT_" + to_string(ix);
    Estimate1DPtr R1;
    book(R1, pre + "_UL", 20, 0., M_PI);
    divide(_h_pT_U->bin(ix), _h_pT_L->bin(ix), R1);
    Estimate1DPtr R2;
    book(R2, pre + "_UC", 20, 0., M_PI);
    divide(_h_pT_U->bin(ix), _h_pT_C->bin(ix), R2);
    pair<double, double> asym1 = calcAsymmetry(R1);
    _h_pT_UL->bin(ix).set(asym1.first, asym1.second);
    pair<double, double> asym2 = calcAsymmetry(R2);
    _h_pT_UC->bin(ix).set(asym2.first, asym2.second);
  }
}
/// @}


/// @name Histograms
/// @{
vector<Histo1DPtr> _h_L, _h_U, _h_C;
Histo1DGroupPtr _h_pT_L, _h_pT_U, _h_pT_C;
/// @}

};

RIVET_DECLARE_PLUGIN(BESIII_2016_I1384778);

} ```